Drive shafts are used in various vehicles, such as automobiles, aircrafts and rotorcrafts. Drive shafts generally connect a rotatable member to an engine that drives the rotation. The engine power rotates the drive shaft, which, in turn, rotates the workpiece. For example, rotorcrafts, such as helicopters, have drive shafts between the rotor, gearboxes and the fuselage-mounted engine(s) to enable the engine to drive the rotation of the rotor blades. The rotation of the rotor blades in relation to the motor creates torque on the drive shaft to transmit power. To ensure safe vehicle operation, the drive shaft must be designed to carry the necessary torque. In addition, the design process must take into account the potential for damage to the drive shaft.
The torque loading on a drive shaft creates internal shear stresses in the drive shaft. The internal shear stresses naturally flow around the outermost closed path of the drive shaft. Thus, in undamaged solid or hollow drive shafts, the internal shear stresses flow around the closed path of the outer perimeter of the drive shaft.
Many of the vehicles that utilize drive shafts are weight-sensitive, such as automobiles and aircraft that have weight limits under which they can safely and/or efficiently operate. Thus, the total weight of the vehicle components and the vehicle payload must comply with the weight limit. In order to increase the payload capacity, the weight of the vehicle components must decrease. In this regard, a drive shaft's weight is generally decreased by making it hollow. A hollow drive shaft's strength is comparable to that of a solid drive shaft while markedly decreasing the weight of the drive shaft. Drive shafts may be made of a range of material, including composites, aluminum, steel, plastic, and wood. In weight-sensitive applications, many drive shafts are made of composite materials that provide a high strength/weight ratio because they are made of layers of high-strength, stiff fibers, such as graphite, embedded in a binder material, such as epoxy resin. The composite materials also offer design flexibility that traditional construction materials, such as metal alloys, do not because the properties of composite materials can be adjusted to efficiently match the requirements of the specific application.
As mentioned above, drive shafts must be able to sustain certain types of damage and retain their ability to carry torque, thereby avoiding catastrophic and/or immediate failure. One type of damage that drive shafts made of composite materials must withstand is damage to the composite materials. The damage may occur accidentally during handling, such as dropping or hitting the drive shaft. While the individual layers of fibers have a very high strength, the adherence between the layers is not as strong and may be damaged by an impact such that the fiber layers or plies separate, which is commonly called delamination. The damaged portion of the drive shaft locally becomes an open cross-section, which decreases the torque load that the drive shaft can carry because the internal shear stresses in that portion of the drive shaft no longer flow about a closed path. This type of damage can be difficult to determine by visual inspection. Without repair, however, the damage will weaken the drive shaft and affect its ability to carry a torque load.
Another type of damage that the drive shaft must withstand is damage from projectiles. For example, the drive shafts of military rotorcraft may experience damage from ballistic projectiles; and the drive shafts of automobiles may experience damage from rocks and other debris that may hit an exposed drive shaft. Experience has shown that ballistic projectiles that hit along the edge of a shaft and create a “slit” along the edge of the shaft can significantly reduce the torsional strength of a shaft, while projectiles that hit in the center of a shaft and create separate entry and exit “holes” only moderately reduce the strength of the shaft.
The damage to the drive shaft may occur when the vehicle is in operation such that the drive shaft must remain operable in order to safely land and/or bring the vehicle to a stop and repair the damage. Safely landing and/or bringing the vehicle to a stop is difficult, however, because, like the damage due to accidents during handling, the projectile damage also creates a locally open section of the shaft, which decreases the torque load that the drive shaft can carry.
Drive shaft designers typically have three options to improve the damage tolerance of drive shafts. The first approach involves increasing the wall thickness of a hollow drive shaft to increase the remaining cross-section of the shaft that carries the torque load after a portion of the drive shaft is damaged. This approach suffers from the drawback that the weight of the drive shaft correspondingly increases, which is what the designers try to avoid. The second approach is to select stronger materials or modify the material properties of the composite, such as by changing the fiber orientation. The increase in damage tolerance of this approach, however, is fairly limited, particularly regarding projectile damage. The third approach consists of increasing the shaft diameter such that the damage area is a smaller percentage of the total area. This approach is often not available to the designer of a drive shaft because of space limitations.
Therefore, a need exists for a drive shaft that is tolerant of accidental and projectile damage to a portion of the drive shaft and able to continue to carry the required torque loads and the corresponding internal shear stresses after the damage. In particular, the need is for a drive shaft that achieves damage tolerance without increasing the radius, wall thickness and/or weight of the drive shaft.